1. Field of the invention
The present invention relates to a time base corrector for treating a composite video signal.
2. Description of Background Information
FIG. 1 shows a basic type of time base corrector using digital memory. As shown in the figure, a reproduced composite video signal having time base fluctuation (jitter), as read from a recording medium such as a video disk and obtained by demodulation, is supplied to this device The received composite video signal is quantized by an A/D converter in a writing clock generation circuit 1C using a writing clock generated in phase synchronization with the composite video signal as a sampling clock. The composite video signal is then written into a digital memory 3 in synchronism with the writing clock. At the time when the composite video signal is written in memory 3, the correspondence of addresses of memory 3 to data written therein is maintained, so that the influence of time base fluctuation is eliminated. The data written in memory 3 is read in synchronism with the reading clock, generated on the basis of a reference clock having a fixed frequency in a reading clock generation circuit 4 and is converted to analog form by a D/A converter 5.
Thus, by writing the data in digital memory 3 using the writing clock synchronized in phase with the input composite video signal and by reading the stored data from said memory 3 using the fixed reading clock, it is possible to eliminate the time base fluctuation component contained in the input composite video signal.
Explanation is further made on the arrangement and the operation of the writing clock generation circuit 1C.
In the writing clock generation circuit 1C, an output of a VCO 11 (voltage controlled oscillator) to be used as the writing clock is divided at a rate of 1/N through a frequency divider 12 and is supplied to a phase comparator 13. The phase comparator 13 performs the sampling of the phase difference between horizontal synchronizing signal, separated and extracted from the input composite video signal in a horizontal synchronizing separation circuit 6 and the output signal of the frequency divider 12. The sampled value is held until the next sampling time and is issued as a phase error signal. This phase error signal is supplied to a control input of the VCO 11 through an LPF (low pass filter) 14.
By the use of the writing clock generation circuit with a PLL (phase locked loop), the writing clock, i.e. output of the VCO 11, is synchronized in phase with the horizontal synchronizing signal of the input composite video signal and is generated as a clock which follows time base fluctuation of the input composite video signal. Accordingly, it is possible to obtain a composite video signal of which the time base fluctuation is eliminated.
However, in the conventional type writing clock generation circuit as described above, the phase comparator 13 detects the phase error only one time in each period T.sub.H of the horizontal synchronizing signal as shown in FIG. 2, and the error value is held until the next detection time. Thus, the circuit is constructed to have a holding time of T.sub.H. Therefore, the error value delayed by the time T.sub.H is produced immediately before the next detection time. By averaging the output signal as a whole, it can be regarded that a delay of T.sub.H /2 is generated.
On the other hand, the response bandwidth of the PLL is determined by the bandwidth of the LPF 14 in PLL system and by the loop gain of the entire system. If it is attempted to broaden the bandwidth of the LPF 14 for improving the response characteristic, the stability of the system will be impaired due to the delay T.sub.H /2 of the phase comparator 13. So, it is impossible to expand the bandwidth as desired. Thus, it is necessary to limit the bandwidth about one tenth or less of the horizontal synchronizing frequency f.sub.H. As the result, a limitation is posed to the follow up capability against the time base fluctuation of the input composite video signal if the phase synchronizing loop only is used, and the performance of time base correction function is limited accordingly.